I was fed up with those preview windows every time I open a new tab on firefox, So I looked into how to get rid of them and found this. Yes Yes you old hands ( linux geeks) will know this but I am posting this for the new people. This will work on both linux and windoze versions of firefox.

HowTo: Turn this feature off completely
In the Location bar, type about:config and press Enter:

Quote:

about:config

The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise!, to continue to the about:config page. Type browser.newtab.url in the search box:

Quote:

browser.newtab.url

Double-click the browser.newtab.url preference and change the url from about:newtab to about:blank Click OK and close the about:config tab.

I was fed up with those preview windows every time I open a new tab on firefox, So I looked into how to get rid of them and found this ... .

There was a privacy issue with that feature : the screengrab-images it uses are big enough that text is (just) readable.
So, for instance, when you visit your online email account a screengrab is made where the subjects of the emails can be readable.
If someone has access to your browser they could look at the Firefox folder for these images of your open email account,
(even though they don't have access to your email account).